This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A variety of protocols are available for the communication of a data message over a communications link. In numerous protocols the message is divided into packets at the transmitter end; the packets are conveyed individually over the communications link; and at the receiver end the packets are combined to re-form the message. Each packet normally comprises a payload, which represents the portion of the message that the packet carries, and control data, which is carried in the packet
One example of a packet protocol is TCP (transmission control protocol). This protocol is widely used as the transport layer protocol in internet communications. The performance of TCP is however significantly reduced in asymmetric data communication systems, wherein a forward channel carries information at much higher speed than a reverse channel.
One such asymmetric system is the DVB-H (Digital Video Broadcasting via Handheld Terminals) system. DVB-H is used to provide high quality video broadcasting services to hand held terminals. It has also been proposed to use DVB-H for downloading game data to hand held devices. DVB-H uses a downlink channel which can transport data from a server to a device at speeds high as 5-30 Mbit/s. When using TCP it is necessary to send acknowledgement packets (ACK) from the device to the server for every data packet received on the downlink channel. The server will only continue to send data upon receipt of ACK packets. The ACK packets are transmitted in the uplink direction on a low bandwidth in a cellular network such as the GPRS network with an average data rate of 10 Kbit/s.
The DVB-H downlink bandwidth is approximately 500˜3000 times greater than the uplink channel which results in a large bandwidth asymmetry. Typically the maximum length of a general TCP data packet sent on the downlink channel is about 1500 bytes, and the length of an ACK packet sent on the uplink channel is 40 bytes. Since the ratio of the packet length between TCP data packet and ACK packet (1500/40=37.5) is much less than the ratio of throughput between downlink channel and uplink channel (500˜3000), this means that the high speed data downloading will cause a large number of ACK packets to be buffered in the hand held device because the data rate of the uplink channel can not satisfy the generating rate of the ACK packets.
As a result of the large bandwidth asymmetry, the blocked ACK packets increase the round trip time (RTT) of the TCP connection and degrade the transmission throughput. Furthermore, the buffered ACK packets will also occupy a lot of resources of the hand held device which will impact on the device performance and other communication processes.
The document “Shekhar et al., Performance Optimisation of TCP/IP over Asymmetric Wired and Wireless links” describes a buffer management solution called SAD (Smart Ack Dropper) for performance optimization of TCP in asymmetric networks. The basic idea of the SAD solution is to monitor the Ack queue status and maintain an Ack sequence number table at the communication node in order to suppress the number of Ack packets belonging to the same flow.
It is an aim of embodiments of the present invention to overcome at least the problems identified above.